1. Field of the Invention
The invention pertains to a field of signal modulation and more particularly to Phase Shift Keying modulation wherein the carrier signal amplitude remains constant during the phase transitions.
2. Description of the Prior Art
Phase Shift Keying (PSK) modulation of a carrier signal as performed in the prior art causes the carrier amplitude to vary during the phase transition stages, thus generating additional frequencies about the carrier over a bandwidth that is a function of the phase transition time.
In general, a PSK signal may be represented by EQU S(t)=A sin [W.sub.1 t+0d(t)]
where:
A is the peak voltage of the transmitted system, PA1 0 is a phase angle that is made to lead or lag by the data d(t)
which takes on the values .+-.1.
In a suppressed carrier, PSK system, 0 is equal to .pi./2, so that S(t) may be represented as: EQU S(t)=Ad(t) cos W.sub.1 t
Since d(t)=.+-.1, it is apparent that the transmitted signals in a suppressed carrier PSK system undergo a 180 degree phase shift with each data transition. These transitions establish a data modulating signal comprising a series of pulses having width equal to the number of data intervals between the phase transitions, as shown in FIG. 1a. Each pulse causes the transmitted signal to exhibit a sin x/x Fourier transform, centered at W.sub.1, which has a spectral width that is a function of the pulse width and weighted by a phase shift term that is dependent upon the pulse position. The formation of these weighted sin x/x distributions is the spectral representation of the transmitted signal.
In the prior art, a signal for transmission as a suppressed carrier PSK modulated signal is generated by coupling the signal to be modulated and the modulating data to a double balanced mixer 11 as shown in FIG. 1b. In some applications, the signal output from the double balanced mixer 11 is coupled to a bandpass filter 12 for the surpression of undesired frequency components, and therefrom to a hard limiting amplifier 13 to insure that a constant level signal is transmitted from the antenna 14. This hard limiting, however, regenerates a significant percentage of the signal frequency suppressed by the bandpass filter.
Though the phase changes are represented as instantaneous in FIG. 1a, in reality the transition is accomplished in a finite time. It is also accompanied by an amplitude variation of the carrier signal during the phase transition period as represented by the regions 15 in FIG. 2a. These rapid amplitude and phase variations generate a wide band of frequencies whch are undesired in terms of transition spectrum usage. It is desired that the modulated signal bandwidth be as narrow as possible to permit a narrow transmission bandwidth, thus providing maximum transmission efficiency. Filtering the data modulated signal in the bandpass filter 12 provides a signal at the input terminals of the amplifier 13 that exhibits a smooth amplitude transition over a longer transition period as represented by regions 16 in FIG. 2b. The amplitude reduction during the phase transition period reduces the signal energy during this period of rapid phase transition. Since most of the frequencies within the wide bandwidth are generated by the rapid phase transitions, reducing the signal energy during this period unless these undesired frequencies. Since it is desired to transmit a constant amplitude signal, the output signal of the bandpass filter 12 is hard limited in amplifier 13 to establish an amplitude envelope between levels 17a and 17b as shown in FIG. 2b. It should be apparent from FIG. 2b, that this hard limiting restores the signal energy during the period of rapid phase transition and, as a result, regenerates signals at the undesired frequencies. A solution to this problem may be realized by slowing down the rate of phase change and maintaining a constant signal amplitude during each phase transition period. A slow phase change rate reduces the generation of the undesired frequencies. With such phase transitions the hard limiter does not generate the undesired frequencies, because it does not alter the phase transition rate when the modulated signal is of a constant amplitude.